Monoclorodifluoromethane (hereinafter referred to as HCFC-22) which has hitherto been widely used as a refrigerant, etc., for an air conditioner, refrigerator, etc., becomes an object of the world-wide regulation as a material of destroying the ozone layer existing in the stratosphere, that is, as a regulated fluorinated hydrocarbon. Thus, pentafluoroethane (hereinafter referred to as HFC-125) having the similar properties as HCFC-22 has been watched with interest as one of substitutes for HCFC-22.
Since HFC-125 is usually produced by reacting perchloro ethane and hydrogen fluoride as raw materials, monochloropentanfluoroethane (hereinafter referred to as CFC-115) is frequently contained in the product as a by-product. However, CFC-115 is also is an object of the world-wide regulation as a controlled fluorinated hydrocarbon (CFC) and must be separated.
As one of methods of separating a fluid mixture into constituting components, a distillation method is the most general method. According to the inventors' investigations, however, the relative volatility of a little amount of CFC-115 to HFC-125 is near 1, for example, the relative volatility is from 1.01 to 1.02, under the pressure of near 5 kg/cm.sup.2 G, and hence it is very difficult to separate HFC-125 from CFC-115 by a simple distillation method.
Under the circumstance, an extractive distillation method of performing a distillation by adding to a fluid mixture, as an extracting reagent, a compound having a different boiling point than the constituting components of the fluid mixture is applied. For example, U.S. Pat. No. 5,082,329 discloses a process of separating HFC-125 from a crude HFC-125 containing CFC-115 by carrying out an extractive distillation using a controlled CFC as an extracting reagent. The extracting reagent is exemplified with 1,2-dichlorotetrafluoroethane, etc. and the extracting reagent itself is a controlled CFC.